Thermal tuning is an easily implementable, low-cost method often employed in electronic and mechanical devices to extract optimal performance or to match the operating points of multiple devices. Similarly, in photonic integrated circuits (PICs), thermal tuners are essential for matching the operating wavelengths of multiple photonic devices such as resonators, gratings, and filters. They are also used for low frequency optical modulation and optical memory applications. The working principle is thermally induced change in refractive index of the materials interacting with guided light.
Conventional thermo-optic tuners are implemented using metal heaters. However, metals absorb near-IR wavelengths, where most of the applications of PICs are, and thus have to be placed a few microns away from the devices to be tuned to avoid degradation in performance of the optical devices. The heating, therefore, is not localized and leads to thermal cross-talk among multiple optical devices. This prevents dense integration of integrated optics. Moreover, these heaters have low power efficiency and a large thermal transient. It is desirable to have a heater closer to the device, yet have it be non-absorbing.
We recently demonstrated that the metallic carbon nanotubes (CNTs) are ideally suited to make these thermo-optic tuners . We fabricated on-waveguide thermo-optic tuners based on solution-processed metallic CNTs on silicon-on-insulator (SOI) and silicon nitride (SiN) micro-ring resonators operating around 1550 nm wavelength. On SOI micro-ring resonators using planarized wire waveguides, a thermo-optic power efficiency of 29 mW/FSR and a thermal transient of 1.3 μs are achieved. The heater is shown to be more power-efficient than conventional metal heaters and has lower thermal transient than both metal heaters and graphene-based heaters. On SiN microring resonators using rib waveguides, improvement in power efficiency with an increase in coverage of CNTs is demonstrated, indicating localized heating using the CNTs; this is favourable for low thermal cross-talk. An optimal power efficiency of 142 mW/FSR and a thermal transient of 5.8 μs are achieved.
The detailed research-findings were accepted for publication in Optical Society of America (OSA) journal Optics Letters 43, 5194-5197 (2018).